Since Ransomware is still all the rage – literally – I decided to write a 4th article with a potentially better method to stop an ongoing infection. In part 1, part 2 and part 3 we focused mostly on detecting an ongoing Ransomware infection and utilized the “nuclear” option to prevent it from spreading: stopping the “server” service which would prevent any client from accessing files on the affected server.

While these methods are certainly effective, there are other more targeted steps you can take instead of or in addition to shutting down the server service, provided that all hosts susceptible to a Ransomware infection are monitored by EventSentry.

When EventSentry detects an ongoing Ransomware infection, it can usually determine the infected user by extracting the domain user name from the 4663 event. Simply disabling the user is insufficient however, since a disabled user can continue to access the network (and wreak havoc) as long as he or she doesn’t log off. Any subsequent log on attempt would of course fail, but that provides little comfort when the user’s computer continues to plow through hundreds or thousands of documents, relentlessly encrypting everything in its path.

As such, the only reliable way to stop the ongoing infection, given only the user name, is to log off the user. While logging a user off remotely is possible using the query session and logoff.exe commands, I prefer to completely shut down the offending computer in order to reduce the risk of any future malicious activity. Logging the user off remotely may still be preferable in a terminal server environment (let me know if you want me to cover this in a future article).

Knowing the user name is of course great, but how do we find out which computer he or she is logged on to? If you have EventSentry deployed across your entire network – including workstations – then you can get this info by querying the console logon reports in the EventSentry web reports. If you are not so lucky to have EventSentry deployed in your entire environment (we offer significant discounts for large quantities of workstation licenses – you can request a quote here) then we can still obtain this information from the “net session” command in Windows.

Net Session Output

We’ve created a little script named antiransom_shutdown.vbs which, given a user name, will report back from which remote IP this user most recently accessed the local server and optionally shut it down. Here are some usage examples:

Find out from which computer boris.johnson most recently accessed this server:cscript.exe C:\Scripts\antiransom_shutdown.vbs boris.johnson

Find out from which computer boris.johnson most recently accessed this server AND shut the remote host down (if found):cscript.exe C:\Scripts\antiransom_shutdown.vbs boris.johnson shutdown

The script uses only built-in Windows commands, as such there is no need to install anything else on the server where it’s run.

When executed with the “shutdown” parameter, the script will issue a shutdown command to the remote host, which will display a (customizable) warning message to the user indicating that the computer is being shutdown because of a potential infection. The timeout is 5 seconds by default but can be customized in the script. It’s recommended to keep the timeout short (5-10 seconds) in order to neutralize the threat as quickly as possible while still giving the user a few moments to know what is happening.

The overall setup of the Ransomware detection is still the same, we’re setting up a threshold filter to detect a higher than usual frequency of certain 4663 events and trigger an action in response. Only this time we don’t shut down the server service, but instead trigger this script. To properly execute the action, configure it as shown in the screenshot below. The executable is cscript.exe (the interpreter for .vbs files) and the command line parameters are the name of the script, $STR2 and “shutdown”.

Remote workstation shut down

So what’s the better and safer approach to freeze an ongoing Ransomware infection? Shutting down the server service is the most reliable approach – since it doesn’t require the workstation to be reachable and will almost certainly succeed. Remotely shutting down a workstation has minimal impact on operations but may not always succeed. See below for the pros and cons of each approach:

Remote Workstation ShutdownPros: Only disables infected user/workstation, even if false positiveCons: Requires workstation to be reachable

This ends up being one of those “it depends” situations where you will have to decide what’s the best approach based on your environment. I would personally go with the remote workstation shutdown option in large networks where the vast majority of workstations are desktops reachable (and not firewalled) from the file server. In smaller, more distributed networks with a lot of laptops, I would go with the file service shutdown “nuclear” option.

A hybrid approach may also be an option for those opting for the remote workstation shutdown method: trigger a remote workstation shutdown during business hours when IT staff is available on short notice, but configure the file service shutdown after business hours when it’s safer and affects fewer people. All this can be configured in EventSentry by creating two filters which are identical except for the action and the day/time settings.

PrerequisitesIt’s important to point out that the EventSentry agent by default runs under the LocalSystem account, a built-in user account which does not have sufficient privileges on a remote host to issue the shutdown command. You can elevate the permissions of the EventSentry agent and work-around this limitation in 2 ways:

Change the service account (fast): Changing the service account the EventSentry service uses to a domain account with administrative permissions will allow the agent to remotely shut down a remote host. This will have to be done on every file server which may issue shut down commands (you can use AutoAdministrator to update multiple file servers if necessary).

Give the “Force shutdown from a remote system” user right: It’s not necessary to issue domain-wide admin rights to the EventSentry agent, the key right the agent needs is just the “Force shutdown from a remote system” user right. The quickest way to deploy this setting is of course through group policy:

a) Open the “Group Policy Management Editor”
b) Edit an existing policy (e.g. “Default Domain Policy”) or create a new group policy
c) Navigate to “Computer Configuration\Policies\Windows Settings\Security Settings\Local Policies\User Rights Assignment”
d) Double-click the “Force shutdown from a remote system” user right and add both “Administrators” and the computer accounts of the file servers to the list. Alternatively you can also create a group, add the file servers to the group, and add that group to the policy (keep in mind that you will need to restart the file servers if you go with the group method).

Once the group policy setting has propagated to the workstations, the remote shut down initiated from the file server(s) should succeed.

Change the “Force shutdown from a remote system” user right

Good luck protecting your network against Ransomware infections, also remember to verify your backups – no protection is 100% effective.

There seems to be a new variant of ransomware popping up somewhere every few months (Locky being the most recent one), with every new variation targeting more users / computers / networks and circumventing protections put in place by the defenders for their previous counterparts. The whole thing has turned into a cat and mouse game, with an increasing number of software companies and SysAdmins attempting to come up with effective countermeasures.

I’ve already proposed two ways to counteract ransomware on file servers with EventSentry in part 1 and part 2, both of which take a little bit of time to implement (although I’d argue less than it would take to restore all of your files from backups). The last part of the series, remediation, offers a way to remotely log off an infected user. In this post I’m proposing a third, and better, method with the following improvements:

In the first article we configured file integrity monitoring on a volume, and if the number of file modifications occurring during a certain time interval exceeded a preset threshold, the ransomware would be stopped in its tracks. In the seconds article we used bait (canary) files to accomplish the same thing.

In this third installment we’ll keep track of the number of file modifications made by a user to detect if an infection is underway. To effectively defeat ransomware, we have to be able to distinguish between legitimate user activity and an infection. To date we know this:

Users add/change/remove files, but the number of changes made by a user in a short amount of time (say 15 min) is generally small

Ransomware always runs in the context of a user, and as such an infection will usually come from one user (unless things go really awry and multiple users are infected). The approach here will work equally well, regardless of the number of infections.

Thus, to detect an infection, EventSentry will be counting the number of file modifications (event 4663) with its advanced threshold capabilities. If the threshold is exceeded, EventSentry will trigger an action of your choice (e.g. disable the user, remove a file share, stop the server service, …) to limit the damage of the ransomware.

Here is what you need:

Object Access / File System Auditing enabled

Auditing enabled on the files which are to be protected

EventSentry installed on the server which needs to be protected

This KB article explains how to configure EventSentry and enable auditing (preferably through group policy) on one or more directories. I recommend referencing the KB article when you’re ready to configure everything. Pretty much everything in the KB article applies here, although we will make a small change to the threshold settings of the filter (last paragraph of section (4)).

Windows Folder Auditing

Once auditing is setup, Windows will log event 4663 for every write access which is performed by a user. An example event looks like this:

Windows Event 4663

The default behavior of a filter threshold in EventSentry is to simply count every filter match towards the threshold. In our case, every 4663 event encountered would count towards the threshold. You can think of there being one bucket for all 4663 events, with the bucket being emptied whenever the threshold period expires, say every 5 minutes. If the bucket fills up we can trigger an alert.

This doesn’t work so well on a file server, where potentially hundreds of users are constantly modifying files. It would take some time to come up with a good baseline (how many file modifications are considered “normal”) that we could use as a threshold, and there would still be a chance for a false positive. For example, a lot of 4663 events could be generated during a busy day at the office, thus causing the threshold to reach its limit.

A better way is to assign each user their own “personal” threshold which we can then monitor. Think of it like each user having their own bucket. If a user writes to a file, EventSentry adds the 4663 event only to that user’s bucket. Subsequently, an alert is only triggered when a user’s bucket is full. Any insertion string of an event can be used to create a new bucket.

We can do this by utilizing the insertion string capabilities of the filter threshold feature. Setting this up is surprisingly easy – all we have to do is change the Threshold Options to “Event”, click the “Insertion Strings” button and select the correct insertion string. What is the correct insertion string? The short answer is #1.

The long answer lies in the “Event Message Browser”, which you can either find through the Tools – Utilities menu in the EventSentry Management Console or in the EventSentry SysAdmin Tools. Once in there, click on “Security”, then “Microsoft-Windows-Security-Auditing”, then 4663. You will see that the number next to the field identifying the calling user (“Security ID”) is %1.

Event 4663 Definition

Enough with the theory, here is what you need to implement it (assuming EventSentry is already installed on the servers hosting the file share(s)):

Customizing the thresholdOnce you have the package & threshold filter for 4663 events in place, we need to modify the threshold settings as explained above. Edit the filter, click the threshold tab and make sure your filter looks like the one shown below:

Threshold Settings

The only variable setting is the actual threshold, since it depends on how fast the particular variant of ransomware would be modifying files. A couple of things to keep in mind:

The interval shouldn’t be too long, otherwise it will take too long before the infection is detected.

Make sure the actual event log filter is only looking at 4663 events, no other event ids.

With the above example, any user modifying any file (on a given server) more than 30 times in 3 minutes will trigger any action associated with the filter, e.g. shutting down the server service. Note that the action listed in the General tab will be triggered as soon as the threshold is met. If 30 4663 events for a single user are generated within 45 seconds, the action will be triggered after 45 seconds, it won’t wait 3 minutes.

Bonus – Disabling a userOne advantage of intercepting 4663 events is that we can extract information from them and pass them to commands. While shutting down the Server service is pretty much essential, there are a few other things you can do once you have data from the events, e.g. the username, available. You can now do things like:

Disabling the user

Removing the user from the share permissions

Revoking access to select folders for the user

There are a couple of caveats when (trying to) disable a user however:

The user account (usually the computer account) under which the EventSentry service runs under (usually LocalSystem) needs to be part of the Account Operators group so that it has permission to disable a user

Disabling a user is usually not enough though, since Windows won’t automatically disconnect the user or revoke access. As such, any ransom/crypto process already running will continue to run – even if the user has been disabled.

Disabling a user account from the command line is surprisingly simple (leave Powershell in the drawer). To disable the user john.doe, simply run this command:

net user john.doe /domain /active:no

Note that since “net user” doesn’t support a domain prefix (MYDOMAIN\john.doe won’t work), we need to make sure that we pass only the username (which is insertion string %2) and the /domain switch to ensure the user is disabled on the domain controller. Of course you would need to omit the /domain switch if the users connecting to the share are local users. The action itself would look like the screenshot below, where $STR2 will be substituted by EventSentry with the actual user listed in the event 4663:

Action to disable a user

That’s it, now just push the configuration and you should be much better prepared to take any ransomware attacks heading your users way.

! Updates !There has been a follow-up post to this article with even better approaches to defeating ransomware. I highly recommend that you jump directly to the most recent article which offers the best & easiest approach for protecting against Ransomware:

When I wrote my first, original post about CryptoLocker (“CryptoLocker Defense for Sysadmins”), I didn’t intend there to be a part 2 or even a part 3. But alas, due to the “popularity” of CryptoLocker and the recent release of CryptoWall 4.0 I decided to write a much-needed sequel to my first blog post. Part #2 differs from the first part with a different (and more simple) detection “algorithm” combined with a more reliable way to stop the “Server” service when CryptoLocker is indeed detected.

Surprisingly (or not surprisingly), almost 2 years after I wrote my first article, CryptoLocker and its descendants like CryptoWall are still around, thriving, and keeping Sysadmins around the world busy. A recent report stated that CryptoWall 3.0 cost victims a combined $325 million, although it fails to mention whether this is an annual or lifetime figure. This is the same as the GDP of the small African country of São Tomé and Príncipe (population of about 200,000) in 2014.

Now there is something to think about – the criminals behind the various ransomware software collected as much money as a country with 200,000 people. Alright, this is all very interesting but doesn’t help us protect ourselves from ransomware so let’s focus.

In part #1 we used EventSentry’s file monitoring feature to index and inventory all files on a susceptible file share, a very accurate and resilient way to detect any sort of software which would modify large numbers of files in a short time period. While this approach works well, it does require more time to setup and may not work in real-time when monitoring extremely large directories. Consequently we’ll be using a different approach here, and we will look at yet another approach in part #3.

EventSentry’s file checksum monitoring feature was originally intended to monitor only key Operating System folders such as the System32 directory, but increased customer demand prompted us to tweak the feature over time to allow real-time monitoring of even very large folders (as is the case for file servers) as well. But enough of the past, let’s tackle Crypto*.

What’s New?
New versions of software (especially free) are usually exciting, but I’m guessing that the latest “improvements” rolled into the various types of ransomware, including CryptoWall, are only exciting for security researches and the people behind the ransomware. There are three major new features included (for free) in the latest version of CryptoWall:

Files are not only encrypted, but file names are now also mangled, making it almost impossible to link the encrypted file(s) with their originals.

Shadow copies are being deleted if possible, so that past versions of files are no longer accessible

The encryption process seems to be less linear and less complete, resulting in some folders being left alone and thus making detection more difficult.

The TheoryIf you’ve been working in the IT (security) field for a while then you’ll have probably heard of honeypots before. Honeypots are usually systems emulating a production server with the purpose of detecting an attacker and potentially triggering counter-measures or alerts.

We’ll apply the concept of honeypots to detect CryptoLocker, but instead of emulating entire systems we’ll plant on or more fake files throughout on one or more file shares with the assumption that any CryptoLocker infection will attempt to change and encrypt those files. Once detected, we can trigger a counter measure such as stopping the server service. The three biggest risk factors with this approach are:

Accidental modification by a user

CryptoLocker detects (and skips) the honeypot files

The bait files get modified too late

But not to worry – we can mitigate all of the risks.

Accidental Modification
Since any unsuspecting user with write access may accidentally modify or delete our bait file, it’s possible that some users curiosity may result in some sort of a accidental DoS attack. Making the file read-only defeats the purpose of detecting CryptoLocker of course, since CryptoLocker itself won’t be able to modify it. I was able to come up with two possible solutions for this problem:

1. Give the file a boring name which discourages users from opening it (e.g. meeting_notes_cl1.docx)
2. Put clear instructions into the file in large font, instructing users not to modify or delete the file. I’d recommend against mentioning any words like CryptoLocker, Virus, etc since CryptoLocker may be parsing the contents of the file.

Honeypot is not sweet enough
Since we don’t have access to CryptoLocker and its constantly evolving code, we don’t know whether it has any honeypot detection capabilities, and if it does, how it attempts to detect them. Since I’m rather safe than sorry, I’m assuming that it has some basic capabilities. It could be as simple as skipping files which are smaller than a pre-defined threshold or looking for specific file names. E.g., based on this article CryptoLocker could now skip any file named meeting_notes_cl1.docx. To maximize our chances for success:

1. Make sure the file is not too small and exhibits properties of other office documents (e.g. 1Mb in size, multiple pages)
2. Give the file a unique, meaningful name, see previous paragraph.

Once you have created the file, place it strategically on your file server among other office documents. I recommend deploying multiple honeypot files if you have multiple file shares. It may be advisable to give the files unique names (e.g. meeting_notes_cl1.docx, meeting_notes_cl2.docx, …) as well.

Too little, too late
The bait file getting modified too late is the biggest risk unfortunately. If you have a directory with 50,000 files but only one bait file, then it won’t help us to detect CryptoLocker. Since we don’t know how CryptoLocker enumerates files (alphabetical, sorted by size, …), it’s probably best to sprinkle them throughout the various vulnerable file shares, using file names which start various letters of the alphabet, e.g.:

a_meeting_notes_cl.docx

m_meeting_notes_cl.docx

s_meeting_notes_cl.docx

z_meeting_notes_cl.docx

A name pattern is not required but helpful when configuring EventSentry later, since it allows you to just specify a wildcard (e.g. *_meeting_notes_cl.docx) instead of specifying dozens of files manually.

Creating multiple bait files is particularly important for newer versions of CryptoLocker which doesn’t always parse/encrypt all directories. So it’s best to create multiple bait files and distribute them across multiple directories, e.g.:

marketing\m_meeting_notes_cl.docx

sales\a_meeting_notes_cl.docx

accounts_payable\z_meeting_notes_cl.docx

This way we’ll have a higher chance of detecting malicious behavior. CryptoWall is fast (of course depending on the speed of the infected host) and can often encrypt tens of thousands of files in an hour.

Implementation
We will use EventSentry’s File Checksum Monitoring feature to monitor the bait files and trigger events when one or more of these files are changed or deleted (=renamed). When they are, we will trigger a script which will stop the server service on the file server in order to avoid more damage being done. Click here to learn more about EventSentry’s architecture.

Monitoring files only for (checksum) changes is no longer sufficient since newer variants of CryptoWall not only modify but also rename (and subsequently delete) documents.

In EventSentry, create a new System Health package with the name “CryptoLocker Detection” and assign it to any server on your network that is monitoring with EventSentry and is serving files through a file share. Now, add the “File Checksum Monitoring” object to the package and ensure the following:

“Monitor folder(s) in real time” is checked

Disable (uncheck) both “Only verify checksum when ..” optimizations

Then, click the “plus” icon to add the (first) folder where a bait file exists to the list of monitored folders. There are a few things to consider when setting this up

The folder/directory name should be specified as it exists on the file server, UNC paths are not recommended.

Check “Detect File Deletions” and “Detect File Checksum Changes”. File size increases and decreases may also be checked but is not required.

Configure the “Files” section to “Only monitor files that are included below” and specify the file name either with a full (relative) path or with a wild card.

Select a severity of “Error” under “Log to Event Log as”

Example
Your file server has a directory called C:\FileShares\Marketing with two sub directories, Ads and Images. If we were to add a bait file to both subdirectories (say specs.docx and meeting1.docx) then we would specify C:\FileShares\Marketing as the folder, and then add

Ads\specs.docx and

Images\meeting1.docx

as the files to be monitored. This is because we always specify the path relative to the main folder being monitored when specifying the file names.

Splendid, EventSentry will now log an event to the event log when any of these files change. Try it out – open the file in word, make a change & save – you should get an alert in the event log almost instantly.

Dive! Stopping the Server Service
Stopping the server service may seem like a drastic step, but it’s unfortunately the most efficient way to prevent an impending CryptoLocker infection from spreading. Sure, blowing up the bridge might seem crazy at first, but if it prevents an army of Zombies (who obviously can’t swim) from entering your town, then we can probably live with the collateral damage.

You can stop a service in 2 ways with EventSentry; with the “Service / Process Control” action as well as with a custom script. Creating a “Service / Process Control” action is easier, but only works for stopping services which have no dependencies. You can probably guess where I’m going with this – the server service depends on other services (e.g. when the “File Sharing Role” is enabled) and thus cannot be stopped with the EventSentry action. Consequently we will go a different route and create a process action instead, which essentially allows you to trigger any process, script etc. Better safe than sorry.

Right-click the Actions container and click “Add” to create a new action called “Stop Server Service”, and select “Process” as the action type. Specify “net.exe” as the Filename, and “stop lanmanserver /yes” as the command line arguments. The “/yes” switch ensures that any service which depends on the “Server” service also gets stopped.

Connecting the dots
Since we now assume that a modification of one or more of our bait files only happens when a CryptoLocker outbreak is under way, the only thing missing now is to have the file change event trigger the process action and shut down the service.

EventSentry uses the concept of “Event Log Filters” to link events to actions, such as sending an email and/or triggering a process. Filters need to be part of an “Event Log Package”, and we can now either create a new package or add our filter to an existing package. For documentation purposes and to keep things orderly we will create a new event log package called “CryptoLocker Prevention”.

We do this by selecting the “Packages – Event Logs” container and clicking “Add” from the ribbon, you can also right-click that container. Give it a descriptive name and select the package, which we now need to assign to one or more hosts and/or groups. Click “Assign” in the ribbon to assign the package, you can also make the package global by clicking the respective button.

With the package all ready to go, we now need to add the filter. With the package still selected, on the ribbon click the “Add” button under “Event Log” and select “Include”. This event log filter, as is, would not apply to any event, since no event log and no severity is selected.

Anything detected by the EventSentry agent (e.g. a file checksum change, service status change, low disk space) is logged to the Application event log with the source “EventSentry”, a matching category (e.g. “File Monitoring”) and usually with a configurable or dynamic severity. In our case the file checksum change events will be logged as Errors, as configured earlier.

So let’s first configure the event properties as shown in the screenshot:

We also add the “Stop Server Service to the list of actions to be triggered. Since we may have other system health packages which log File Monitoring events, we want to make sure that this filter only applies to those, which we do by restricting the filter further with an event id as well as with a Content Filter.

For CryptoLocker we want to get notified about every change that happens to our bait file. Whether it’s deleted, a checksum change or a file size change. As such, we leave the event id field empty and specify the “File Monitoring” category instead.

Important Note: If you are running a German version of Windows, the category will need to be specified in German (“Dateiüberwachung”) since EventSentry is localized for German.

Our filter could still apply to unrelated file checksum changes (e.g. OS files were changed by a Windows Update), but since any file checksum change event includes the package name which triggered the event, we can filter based on that name (we called the package “CryptoLocker Detection”) to ensure that we only match file changes from CryptoLocker. In the “Content Filter” section click the “+” button to add a new content filter.

The quickest way to specify the content filter is to leave the “Wildcard match” setting in place and simply specify *CryptoLocker Detection* as the content filter. A more elegant way is to use an Insertion String match and selecting insertion string 5, which represents the package name (click “Preview” to see the insertion string numbers).

The setup is now complete, and you can now push the configuration to the remote host(s) which has the bait files and should be protected. If you have multiple file servers with a different directory structure, then you can easily create multiple system health packages which contain a file monitoring object, and assign them accordingly. For example, you could create packages named:

CryptoLocker Detection Server1

CryptoLocker Detection Server5

The process action doesn’t have to be duplicated, since the stopping the service is the same process for all hosts. The event log filter may need to be adjusted depending on how it was setup. A wild card like *CryptoLocker Detection* would match “CryptoLocker Detection Server5” as well, but an insertion string filter would need to be modified to something like CryptoLocker Detection* in order to match multiple more than one package.

! Updates !There have been 2 (!) follow-up posts to this article (part 2 and part 3), with even better approaches to defeating ransomware. I highly recommend that you jump directly to the most recent article which offers the best & easiest approach for protecting against Ransomware:

It seems as if CryptoLocker has been making the rounds lately, much to the dismay of users who don’t have working backups of their precious office documents.

While I admire Cryptolocker’s simplicity and effectiveness from a purely technical and entrepreneurial standpoint, what the software is doing does appears to be illegal in most countries and so I’d like to offer some advise on how to tame the beast. If you’re looking for a 5-minute fix then I have bad news: implementing the CryptoLocker defense I have outlined below, while completely free, will take a little more than 5 minutes to implement. But knowing that you have an effective defense against CryptoLocker may very well be worth it. After all, CryptoLocker seems to find its way into a lot private networks these days.

The ideas set forth in this blog post apply mostly to Windows-networks with file servers, but could be adapted for individual computers as well (though this is not covered here – let me know if you’d like me to include this scenario).

About CryptoLocker
For those who have not heard of CryptoLocker yet, it is a piece of software which encrypts pretty much all common office-type documents, including Microsoft Office, AutoCAD, PDFs, images and more. This blog article from MalwareBytes has a complete list of extensions. Once encrypted, CryptoLocker charges you to decrypt (your own files) again. It’s public key cryptography gone wrong; I wonder if Diffie & Hellman saw this one coming. And to make the whole spiel even more interesting, you only get a limited amount of time to pay before your files will remain encrypted. Forever. Oh – and the longer the wait, the more you have to pay. And with recent bit coin exchange rates in excess of USD 1000, the amount that needs to be paid can be uncomfortably high.

It is pretty difficult to defend against something like CryptoLocker other than through usual means of AntiSpyware software, user eduction and strict policies against opening and downloading files from the Internet, email attachments and such. In most cases CryptoLocker comes in form of a ZIP attachment disguised with a PDF icon.

One reason CryptoLocker is so effective – yet difficult to block – is because it exhibits the same behavior as users would: It “simply” accesses and modifies files like a user would. And infecting a machine isn’t all that difficult since CryptoLocker doesn’t require any elevated permissions to run. On the contrary, it wants to run in the same context the user does, so that it can access and see the same files a user does. As such, security features like UAC are utterly useless against ransomware like CryptoLocker – it’s a whole new type of software.

Backups
The most effective defense against CryptoLocker is to have a working, tested backup. Let me repeat this: A WORKING and TESTED backup. Users have lost all their data because they thought that they had a backup in place when their backup was broken in some way.

We’ve seen posts of users who deleted all the files CryptoLocker encrypted, thinking they had a working backup. They had a backup, but it was apparently not recently tested and as a result the user lost all of their data.

Naturally, CryptoLocker does not like backups. It dislikes them so much that when CryptoLocker runs, it even tries to delete any Windows Shadow Copy backups. Cloud backup services (including Dropbox, Skydrive and Google Drive etc.) which keep versions of your files offer some protection, but restoring older versions of your files may be a tedious process.

The DefenseThe most obvious defense against CryptoLocker is AntiSpyware software, e.g. MalwareBytes. Most AntiSpyware & AntiVirus software still uses signatures however, so new versions of the ransom ware often remain undetected at least for a few days.

So instead of detecting CryptoLocker itself, we can sniff its tracks so to speak. CryptoLocker’s predictable behavior can be used against it. CryptoLocker’s objective is of course to encrypt and hold hostage as many files as possible, so to increase the likelihood of the user purchasing the decryption key from the thugs.

And it is that very pattern that we will try to exploit and use as a trigger to detect and take corrective measures. The approach consists of measuring how many files are being changed in a certain time interval, and if a certain threshold is being exceeded (say more than 10 files modified in 1 minute) we assume that CryptoLocker found its way into our castle. Even though users modify their documents on a regular basis, users can usually make only so many changes at a time and most likely at a much slower rate than any sort of script / software would.

Another approach would be to create one or more honeypot or canary files, which we know (or hope) a user would not modify. If a checksum change in one of those files were detected, we could (more or less) safely assume that CryptoLocker was on one of his rampages again and take corrective measures. The honeypot file would have to be modifiable by users (otherwise CryptoLocker would also not be able to modify it), which makes accidental modifications by users possible (although somewhat unlikely).

This 2nd approach isn’t quite as solid in my opinion, since CryptoLocker is most certainly adapting to changes, and may skip files that it may suspect are a trap. For example, it could skip small files or skip directories with a very small number of files and so forth.

A more sophisticated approach, where we detect an unusually large number of files changes in a small time period, is going to be harder to circumvent by CryptoLocker. The good news is that we have a free (it’s really free, not a trial) software tool available which can do just that. It can:

detect file changes

measure the rate of file changes (through event log alerts)

stop/start services or launch processes

send out alerts

EventSentry (Light) to the rescueEventSentry Light is the free version of our full-spectrum monitoring & compliance solution EventSentry. The features we can utilize to come up with a defense are:

File Checksum Changes (part of System Health Monitoring)

Filter Thresholds (part of Event Log Monitoring)

Action (control services, send out emails)

File Checksum Monitoring
Monitors any folder and detects file size changes, checksum changes and file additions and deletions. EventSentry Light will log file checksum changes to the event log (it’s big brother can also log them to a database), which in turn is monitored by the real-time event log monitoring component.

Event Log Monitoring & Thresholds
This component supports a variety of sophisticated features, one of which are thresholds. The thresholds feature lets you essentially detect event log entries that occur at a certain pace. For example, if 10 specific events occur in 1 minute then let me know and/or take corrective action.

Service Action, Email Action
EventSentry supports a variety of action types to be triggered when an event occurs, with email usually being the most commonly used one. You can also control services, use REST APIs, launch processes and much more. We’ll use the former to stop the file sharing services (LanmanServer) when we have determined that CryptoLocker is on the loose.

I will go into step-by-step instructions on how to configure EventSentry at the end of the post.The Baseline
The most difficult thing to determine is the maximum rate of file changes we deem normal, as we need to have a baseline in order to configure the threshold slightly above that. This number will vary from network to network, with file servers serving lots of users obviously requiring a larger threshold. I’d like to repeat that determining the right threshold is very important. If it is too low, then normal user activity will trigger an alarm; if it is too high then the alarm may never be triggered and CryptoLocker won’t be caught in time.

The best approach is to setup file monitoring and let it do its job for 1-2 days to determine a baseline. Once the baseline is established, we can increase it by a certain factor (say 1.5) and use that as the threshold.

Setting up the trap requires 3 steps. In this case we assume that EventSentry is either installed directly on the file server, or an agent is deployed on the file server (in which case you will need to make sure that configuration updates are pushed to the file server(s) in question).

Step 1: Monitoring the directory/ies
In EventSentry, right-click the system health packages and add a new package. Right-click the package, select “assign” and assign it to all file servers. Right-click the package again and add a “File Monitoring” package. Click the new object. Directories are monitored in real time by default, but EventSentry requires a recurring scan as well – in case Windows doesn’t send real time notifications. This is usually a good thing, but when you are monitoring large directories it’s best to set the interval very high (future versions will allow for this to be unchecked).

In the package, add all the folders which should be monitored and only check the “checksum change” check box. Do not check any of the other check boxes in the bottom left section at this time. Since we haven’t established a baseline yet, we’ll set the severity of the event log alerts to “Information”. If the monitored folders contain a lot of non-Office files then it may be a good idea to adjust monitoring so that only office files (e.g. .doc, .xls, etc.) are monitored. If you prefer to monitor all files, simply change the setting to the green PLUS icon and make sure the list of exclusions is empty (or specifies files that should be excluded, e.g. *.tmp). Below is a screenshot of how this can be configured.

When you save the configuration, EventSentry will enumerate all files in the folder and create an initial checksum for every file. The agent will log event 12215 when the scan starts, and event 12216 when the scan is complete. When that happens, EventSentry is essentially “armed” and will detect, and log, all checksum changes to any of the files in the monitored directories.

At this point we’ll want to let this run for at least 24 hours during a “normal” work day, as to determine how many file changes occur on average. You are going to be at a bit of an advantage if you are running the full or the trial version with database support, as it will be a lot easier to determine the number of file changes occurring through the web-based reporting.

Step 2: Setting up the trap
Now that we have established a baseline, we’re ready to setup a threshold. This time we’ll create a new event log monitoring package. Right-click “Event Log Packages” and add a new package and call it “CryptoLocker Rules”. Like before, assign it to the file servers we are monitoring. Right-click the package again and add a new event log filter. Configure the filter as shown in the screenshot below. Note that we are triggering an email action for now. The content filter can be used to restrict the filter further, e.g. to only match certain directories if you are monitoring several directories with EventSentry.

Now things are getting interesting. The goal is to create an error event in the event log when X amount of file checksum changes occur in a given time period. To get there, we’ll start with the “General” tab where we tell the filter what type of event we are interested in (see below). Once that event is defined, we’ll move on to the “Threshold” tab which is where we specify the threshold parameters. For the purpose of an example, let’s assume that we have established a baseline of 100 file checksum changes per day, with a work day starting at 8am and ending at 7pm. Assuming that activity is somewhat spread throughout the day, this amounts to about 9 file changes per hour. Naturally we’ll have to assume that file changes aren’t always evenly spread out throughout the day, but setting up a “if 20 checksum changes occur in 1 minute shut file sharing down” is probably a reasonable threshold. Configure the threshold as shown in the screenshot below, with whichever threshold you came up with.

Step 3: Triggering corrective action
When our threshold is reached, EventSentry will log an error to the event log with event id 10601 and trigger the specified action(s) from the “General” tab (Default Email) one time per threshold interval.

At this point we would merely receive an alert when we suspect that CryptoLocker is at it again. If you are cautious then you can retain this setup for a little while (e.g. a day or two) to ensure that you are not getting any alerts about the threshold being met (assuming that CryptoLocker is not active on your network in which case you should get the emails).

To go all in and trigger a server service shutdown, we’ll need to create a service action now. On Windows, file sharing services are provided by the “Server” service, which uses the internal name of “LanmanServer”. The service action allows you to control any service (start/stop/restart), and in this case we’ll obviously want to stop the server service, so that clients cannot access the file shares on your server anymore. We’ll trigger an email action at the same time of course, so that the sysadmin in charge is aware of what is going on. While shutting down all file services seems a bit extreme, it’s unfortunately the most effective way to prevent more files from becoming encrypted.

So for the next step, right-click the “Actions” container and select “Add Action”. At the selection dialog choose the “Service” action, enter a descriptive name (e.g. “Stop File Sharing”) and hit enter.

Then, configure the settings of the service as shown in the screenshot below.

The last step of our setup (congratulations if you’ve made it that far) is to assign the service action to the filter we previously created. After all, a service action which isn’t referenced anywhere doesn’t do much good. So head back to the Event Log Packages, find the “CryptoLocker Rules” package and edit the filter in the package. In the action list on top, click the “Add” button and add the action you just created.

Testing
If at all possible I’d recommend testing the EventSentry setup at a time when your users are not interrupted. Adding a few template files to one of the monitored folders and changing them in short succession (a script may be necessary depending on how short your threshold interval is) should trigger the file services shutdown procedure. Once verified, you can just start the “Server” service again.

Conclusion
Just like in the real world, network viruses come in all shapes and sizes – only limited by technology and the imagination of the cyber-evildoers.

I hope that this article gave you some insight into CryptoLocker and a good way to guard against it. As always, make sure that your company has the following in place:

Email Attachment scanning

Working, tested backups

User education

AntiSpyware software

With those in place, one should be able to keep future infections to a minimum.